Flammable and otherwise hazardous materials play an important role in the everyday lives of most people. Most people encounter flammable materials, such as gasoline, engine oil, and natural gas, and other hazardous materials, such as battery acid and concentrated detergents, without danger. Because the unsafe materials are contained, they typically present no problem for those that are nearby.
When the unsafe materials become uncontained, however, the materials can injure or kill, such as when the container is damaged and the material escapes. For example, hundreds of thousands of vehicular accidents occur each year on American highways. Many accident-related fire events occur when the region of the vehicle containing the fuel tank is impacted in an accident, spilling the fuel contents from the tank in the form of a spray, stream, and eventual pool around the vehicle. The highly ignitable spray mist generated upon impact may be exposed to ignition energy from sparks generated from vehicle deformation on impact for only a fraction of a second. This duration, however, may be long enough to ignite the fuel mist into a possible explosion, or more likely a fireball that ignites a developing pool of fuel surrounding the vehicle and create a more serious threat.
In many cases, the threat of ignition and resultant flame spread only exists for the instant that the sparks from the impact event remain. These events have been noted particularly on several recent automotive and truck designs that were hypothesized, due to tank placement and structural design, to have potentially higher rates of incidences of such events. These high profile examples often lead to spectacular fire events and the higher rates of burn injuries and fatalities when they occur, and have resulted in national discussions on how to prevent their continued occurrence.
Unfortunately, most fire protection technologies are impractical for general highway vehicle or other consumer use, due to cost, complexity, reliability problems, and substantial weight increases. As a result, little has been done to prevent such events in the future. The military, however, has confronted similar events that occur in combat scenarios. In particular, military aircraft that are impacted by anti-aircraft projectiles can develop fires in adjoining bays adjacent to fuel tanks onboard the aircraft. The fuel leaking or spraying from a penetrated tank encounters ignition sources, such as burning incendiary particles deposited by the projectile in the adjoining bay, with resultant fires threatening the interior of the aircraft. Many aircraft losses in combat have been attributed to such events.
As a result, technologies have been developed in recent decades to prevent or suppress such events for newer combat aircraft. One approach to aircraft fire protection uses passive systems. These systems are typically some form of structure that requires no electrical power or other artificial monitoring. These systems function by being impinged directly by the explosion or fire event. They typically provide explosion protection inside the fuel tank or in surrounding compartments around the fuel tank. One of the earliest and most successful variants was the use of flexible reticulated foam in fuel tanks to mitigate explosions. This concept was extensively used successfully in the latter stages of the Vietnam War and became a fixture on many modern era aircraft.
The British military developed several advanced concepts in the early 1970s. These included forming reticulated foam into balls to fill various compartments adjacent to fuel tanks in aircraft (U.K. Patents 1,380,420, 1,445,832, and 1,454,492) that could be coated with substances that swell upon heating to cut off air supply to the fire, and filled with various gaseous and powder extinguishing agents to provide extra fire extinguishing in addition to fire mitigation. The main advantages of such concepts were ease of installation, high reliability due to lack of sophisticated electronics and other devices, and competitive weight penalties in comparison to active fire suppression systems, such as gaseous fire extinguishing and detection systems, with the trade-off depending upon the compartment volume and configuration.
Other passive protection systems use fire suppressants embedded into rigid or semi-rigid panels mounted onto the wall of the fuel tank adjoining and facing an adjacent bay. The panels, when impacted by a projectile penetrating through the aircraft, would rupture locally and release a portion of suppressant into the adjacent bay, extinguishing the beginnings of fuel spray from the damaged fuel tank entering the bay and igniting, or rendering the fuel vapors inert against ignition when coming into contact with the deposited incendiary particles. The panels were developed and demonstrated with gaseous extinguishing agents and various powders (U.K. Patents 1,454,493 and 1,547,568). The panels took the form of hollow panels with cylinders or sachets of suppressant inserted, or balls or sheets of reticulated foam (sometimes sealed in bags with a pressurized gaseous suppressant).
All of these variations showed some level of performance enhancement for a given system volume or weight, but could be offset by increased complexity or increased material, assembly, or installation cost. The most common and simple variations were thin panels with a hexagonal honeycomb sandwich material of kraft paper, aluminum, or Nomex, filled with a fire extinguishing powder and covered with a thin sheet on both faces of aluminum foil, composite fibers, or other materials. These devices were described as powder panels or powder packs.
The powder panels were demonstrated to effectively protect against many large ballistic incendiary threats with as little as 0.1 inch total thickness and 0.2-0.6 pounds mass per square foot. Other threats and conditions could require much thicker, heavier, systems if they worked at all. Some limitations in performance were seen against small threats that limited rupture damage to the panel and as a result limited the amount of powder suppressant released to extinguish the fire.
Variations of this concept were investigated for use against ballistic impacts in armored vehicles (U.S. Pat. Nos. 3,930,541 and 4,132,271), although powders were primarily limited for use in engine compartments due to the inhalation difficulties with crew members, and gaseous suppressant filled panels were used in the crew compartment. Later fine tuning was made including adding spall shields to prevent spallation damage from the panels to crew members.
Since these systems require ballistic impact to function, their utility and consideration was limited to combat-induced ballistic impact events; they offer no protection against gradual fuel system leakage and ignition due to ordinary fuel system failures. Further, such systems do not provide protection against other types of threats or problems. For example, such systems do not provide protection in other fire scenarios, such as collisions impacting and fracturing fuel tank valves and their connectors, particularly for alternate fueled vehicles. Additional flammable fluid reservoirs, such as brake master cylinders and fuel pumps, contain sufficient flammable fluid to pose a threat to vehicle occupants or the vehicle itself, and their small, bulky shapes provide difficulties in providing protection. Other areas of a vehicle, such as the vehicle's engine compartment hood, exhibit damage in front end crashes, and may cause the release of flammable or otherwise hazardous materials. Further, some components, such as the oil pan, may rupture and discharge flammable fluids due to the internal destruction of the engine, which is typically accompanied by the fracturing and penetration of the connecting rods through the oil pan. This scenario is very common in automobile racing in addition to highway occurrences.